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It is true that higher ac voltages are beneficial for ac power transmission from an efficiency point of view but there are other restrictions as well. The transmission system designed for the transmission of ac power at a particular voltage must take into account various factors such as:

Ground Clearance: The distance between the lowest point of a transmission line and the ground is referred to as ground clearance. It depends on the voltage to be transmitted via the line. Higher the voltage, higher the clearance required. This means the height of transmission tower required to transmit the power will also be more and so is the material required for construction. Also, a very large height of towers is subjected to various mechanical stresses that may require special support to stand erect in all climatic conditions. All this adds to the cost of transmission and therefore such a system is highly uneconomical.

Insulation: Higher voltages require high insulation. For low voltage transmission system air acts as an insulation medium and thus saves cost but much higher voltage system may ionize the air jeopardizing the system and surroundings.

A clearance between two phases/lines: Higher voltages will require higher clearance between two lines which increases the width of a tower. This is also uneconomical as explained in point 1.

    Minimum Electrical Clearance
    Voltage in KV Phase to earth in mm Phase to phase in mm
    6.6 139.7 177.8
    11 177.8 228.6
    22 279.4 330.2
    33 381 431.8
    66 685.8 787.4
    110 863.6 990.6
    132 1066.8 1219.2
    220 1778 2057.4

    Therefore a compromise is made between efficiency and associated cost of the transmission to provide satisfactory results.
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